Method of Producing Elemental Phosphorus and Thermal Phosphoric Acid
Elemental phosphorus is produced commercially by smelting a mixture of phosphate ore, reducing carbon, and silica rock in submerged-arc electric furnaces. Metallurgical coke is commonly used as reducing carbon but various other forms of carbon can be used. In the furnace, carbon combines with oxygen in phosphate ore to form carbon monoxide, and phosphorus is released as the element. Also, carbon combines with water to form both carbon monoxide and hydrogen by the following chemical reaction. EQU C+H.sub.2 O=CO+H.sub.2.
Feedstock is normally dried, but some moisture remains after drying and this provides water for reduction reaction shown above. Feedstock may contain combined water which will not be removed by drying. Combined water is reduced to form carbon monoxide and hydrogen.
Silica rock supplies SiO.sub.2 which combines with CaO and Al.sub.2 O.sub.3 in phosphate ore to form calcium silicate and aluminum silicate. Mixture of silicates is removed from the furnace as a molten material by tapping. Mixture is called slag, and various metals which are found in phosphate ore are incorporated in slag as an insoluble and innocuous mass. Sometimes low-grade phosphate ores are smelted which contain enough SiO.sub.2 to combine with CaO and Al.sub.2 O.sub.3 and in this case no silica rock is used in the feed mixtures.
Gases discharging from furnaces are a mixture of carbon monoxide, hydrogen, and elemental phosphorus. The mixture is called furnace gases. Elemental phosphorus is separated from gas mixture by cooling the furnace gas by contacting it with water. Elemental phosphorus condenses as a liquid; however, a phosphorus-containing solid is obtained which is called phosphorus sludge. Although phosphorus sludge is normally treated to recover elemental phosphorus, recovery of the element is incomplete and a phosphorus-containing solid is obtained. Since elemental phosphorus is a toxic chemical, phosphorus-containing solids are hazardous substances.
At some plants, dust is removed from furnace gases before they are cooled with water. Electrostatic precipitators may be installed to remove dust, and these devices are upstream from the phosphorus condenser. A small amount of elemental phosphorus adsorbs on the dust particles and collected dust is a phosphorus-containing solid. However, electrostatic precipitators decrease the quantity of phosphorus sludge made.
When furnace gases are contacted with water they are cooled adiabatically. Particles of inorganic solids, including particles of elemental phosphorus, become suspended in the water. A process was developed to remove suspended solids from the condenser water and thereby permit the water to be reused as a cooling medium. In the process, suspended solids were separated from liquid phase by clarifying condenser water as described in a publication, "Waste Effluent; Treatment and Reuse," J. C. Barber, Chemical Engineering Progress, volume 65, No. 6, June 1969. Overflow from clarifier was reused for cooling furnace gases and underflow was a phosphorus-containing solid. Clarifier underflow is another hazardous waste generated when elemental phosphorus is produced.
Dissolved solids accumulate in condenser water but these solids are not removed by clarification. Dissolved solids may be fluorine compounds and ammonium phosphates and they will continue to accumulate in recycled condenser water until the water becomes ineffective as a cooling medium. Some of the fluorine compounds are present as fluosilicate salts which may precipitate as tenacious scales in the condensing equipment. Therefore, a stream of clarified condenser water is bled off and replaced with water containing small concentrations of dissolved solids. Rate of bleedoff is usually determined by buildup of fluorine compounds. Condenser water contains elemental phosphorus in forms which are not removed by clarification; consequently, bleedoff is a phosphorus-containing liquid frequently called "phossy water." It is toxic waste because it contains phosphorus in its elemental form.
Hazardous wastes generate during production of elemental phosphorus are as follows.
Precipitator dust PA1 Phosphorus sludge PA1 Clarifier underflow PA1 Phossy water
About 80 percent of the phosphorus produced in the United States is converted into a product called thermal phosphoric acid. In the conversion, phosphorus is burned with air to form P.sub.2 O.sub.5 and this oxide then combines with water to form phosphoric acid.
Much energy in the form of electric energy and reducing carbon is required to smelt phosphate ores. However, much of the energy is released when elemental phosphorus is converted into phosphoric acid. Heat is released when elemental phosphorus is burned to form P.sub.2 O.sub.5, and additional heat is released when P.sub.2 O.sub.5 reacts with water to form phosphoric acid. Heretofore, energy from conversion of elemental phosphorus into phosphoric acid has been wasted, but processes are needed to convert wasted heat into electric energy.
the phosphorus industry has been severely handicapped because of the following problems.
1. Rigid enforcement of environmental regulations threatens the industry because of cost of environmental controls.
2. Electric energy costs have skyrocketed and this has caused large increases in cost of producing elemental phosphorus.
There has been little incentive for development of new products from either elemental phosphorus or thermal phosphoric acid.
A program was undertaken to provide relief from the two problems cited above. Processes were developed to reuse phosphorus-containing solids and to recover phossy water in suspension fertilizers. Energy is recovered in the form of electric energy. Disclosures presented herewith illustrate processes and equipment for correcting the hazardous waste problem and for conserving electric energy.